This invention relates to improvements in a windmill-shaped electrode used with vacuum circuit interrupters.
Vacuum circuit interrupters are now becoming important in the field of AC high voltage circuit interrupters and include generally a magnetic drive type electrode which is called a windmill-shaped electrode. Among the various excellent features thereof vacuum circuit interrupters have the important merit that they can be made small-sized. However it can not be said that vacuum circuit interrupters with a rated interrupted current value in excess of 40 kiloamperes are sufficiently small-sized as compared with conventional oil circuit interrupters having a small amount of oil and conventional gas-blast circuit interrupters utilizing gaseous sulfur hexafluoride (SF.sub.6). Particularly, the interrupting portion thereof which has a large diameter has been one of impediments in the increased use of vacuum circuit interrupters in the field of high current capacities. On the other hand, vacuum circuit interrupters are still expensive among small-sized circuit interrupters having a rated interrupted current value on the order of 8 kiloamperes and a fairly large proportion of this is attributable to the windmill-shaped electrode disposed in such circuit interrupters.
The windmill-shaped electrode used in vacuum circuit interrupters includes the central circular flat portion having a contact function and a tapered portion surrounding the central flat portion which is windmill-shaped and having a plurality of circular arc-shaped slots radially and circumferentially extending therethrough thereby to drive magnetically an electric arc which strikes the electrode.
For conventional windmill-shaped electrodes there has not yet been established an approach to their design geometry such as the radius of curvature of and angle subtended by the circular arc-shaped slots at the centers thereof, the number and width of the slots and the shape of the tips of the blades forming the windmill etc. Accordingly, the entire surface of such windmill-shaped electrodes has not been effectively put to practical use for achieving the interruption of current and therefore the interrupting current to which can be interrupted has been comparatively low although the electrodes have a comparatively large maximum radius. For example, since the circular arc-shaped slot has been too small has too large a radius of curvature, the circumferential or radial length thereof has been insufficient and causes the magnetic driving effect to be excessively small or deficient. This might selectively melt the tips of the windmill portion or the central flat portion of the windmill-shaped electrodes thereby to make it impossible to interrupt the particular current. Also, because the circular arc-shaped slots have been, for example, excessively narrow in width, a portion or portions of the electrode melted at the time of interruption might electrically shortcircuit the slot or slots resulting in a failure to interrupt the current involved. On the contrary, when the slot width is large enough to cause the surface area of the windmill-shaped portion to be insufficient, this might also result in a decrease in interrupting capacity. Further, because the blades forming the windmill have an excessively large weight, it has been required to increase the mechanical strength of the root of each blade. Consequently, a thicker structure has inevitably resulted. Thus conventional windmill-shaped electrodes have been so complicated in structure that, for example, each of the circular arc-shaped slots might be formed of a plurality of circular arcs having different radii of curvature and/or different centers and merged into one another. Further the electrodes have been thick. Accordingly, windmill-shaped electrodes of the conventional construction have been disadvantageous in that the circular arc-shaped slots can not be easily machined, the wear and tear on the machine tools for machining such slots are severe and the machining time is long.
Accordingly, it is an object of the present invention to provide a new and improved windmill-shaped electrode permitting the resulting vacuum circuit interrupter to be small-sized.
It is another object of the invention to reduce the cost of vacuum circuit interrupters by provision of a new and improved windmill-shaped electrode which is easy to machine.